This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present disclosure. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present disclosure. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Field Of The Invention
This invention relates generally to the field of wellbore operations. More specifically, the invention relates to completion processes wherein multiple zones of a formation are fractured along a wellbore in a seamless manner.
General Discussion Of Technology
In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. After drilling to a predetermined depth, the drill string and bit are removed and the wellbore is lined with a string of casing. An annular area is thus formed between the string of casing and the surrounding formations.
A cementing operation is typically conducted in order to fill or “squeeze” the annular area with columns of cement. The combination of cement and casing strengthens the wellbore and facilitates the zonal isolation of the formations behind the casing.
It is common to place several strings of casing having progressively smaller outer diameters into the wellbore. A first string may be referred to as surface casing. The surface casing serves to isolate and protect the shallower, fresh water-bearing aquifers from contamination by any other wellbore fluids. Accordingly, this casing string is almost always cemented entirely back to the surface.
The process of drilling and then cementing progressively smaller strings of casing is repeated several times until the well has reached total depth. In some instances, the final string of casing is a liner, that is, a string of casing that is not tied back to the surface. The final string of casing, referred to as a production casing, is also typically cemented into place. In some completions, the production casing (or liner) has swell packers spaced across the productive interval. This creates compartments between the swell packers for isolation of zones and specific stimulation treatments.
As part of the completion process, the production casing is perforated at a desired level. This means that lateral holes are shot through the casing and the cement column surrounding the casing. The perforations allow reservoir fluids to flow into the wellbore. Thereafter, the formation is typically fractured. In the case of swell packers or individual compartments, the perforating gun penetrates the casing, allowing reservoir fluids to flow from the rock formation into the wellbore along an individual zone.
Hydraulic fracturing consists of injecting water with friction reducers or viscous fluids (usually shear thinning, non-Newtonian gels or emulsions) into a formation at such high pressures and rates that the reservoir rock parts and forms a network of fractures. The fracturing fluid is typically mixed with a proppant material such as sand, ceramic beads, or other granular materials. The proppant serves to hold the fracture(s) open after the hydraulic pressures are released. In the case of so-called “tight” or unconventional formations, the combination of fractures and injected proppant substantially increases the flow capacity of the treated reservoir.
In order to further stimulate the formation and to clean the near-wellbore regions downhole, an operator may choose to “acidize” the formations. This is done by injecting an acid solution down the wellbore and through the perforations. The use of an acidizing solution is particularly beneficial when the formation comprises carbonate rock. In operation, the completion company injects a concentrated formic acid or other acidic composition into the wellbore, and directs the fluid into selected zones of interest. The acid helps to dissolve carbonate material, thereby opening up porous channels through which hydrocarbon fluids may flow into the wellbore. In addition, the acid helps to dissolve drilling mud that may have invaded the formation.
Application of hydraulic fracturing and acid stimulation as described above is a routine part of petroleum industry operations as applied to individual hydrocarbon-producing formations (or “pay zones”). Such pay zones may represent up to about 60 meters (100 feet) of gross, vertical thickness of subterranean formation. More recently, wells are being completed through a producing formation horizontally, with the horizontal portion extending possibly 5,000, 10,000 or even 15,000 feet.
When there are multiple or layered formations to be hydraulically fractured, or a very thick hydrocarbon-bearing formation (over about 40 meters, or 131 feet), or where an extended-reach horizontal well is being completed, then more complex treatment techniques are required to obtain treatment of the entire target formation. In this respect, the operating company must isolate various zones or sections to ensure that each separate zone is not only perforated, but adequately fractured and treated. In this way the operator is sure that fracturing fluid and proppant are being injected through each set of perforations and into each zone of interest to effectively increase the flow capacity at each desired depth.
The isolation of various zones for pre-production treatment requires that the intervals be treated in stages. This, in turn, involves the use of so-called diversion methods. In petroleum industry terminology, “diversion” means that injected fluid is diverted from entering one set of perforations so that the fluid primarily enters only one selected zone of interest. Where multiple zones of interest are to be perforated, this requires that multiple stages of diversion be carried out.
In order to isolate selected zones of interest, various diversion techniques may be employed within the wellbore. Known diversion techniques include the use of:                Mechanical devices such as bridge plugs, packers, down-hole valves, sliding sleeves (known as “frac sleeves”), and baffle/plug combinations;        Ball sealers;        Particulates such as sand, ceramic material, proppant, salt, waxes, resins, or other compounds;        Chemical systems such as viscosified fluids, gelled fluids, foams, or other chemically formulated fluids; and        Limited entry methods.        
These and other methods for temporarily blocking the flow of fluids into or out of a given set of perforations are described more fully in U.S. Pat. No. 6,394,184 entitled “Method and Apparatus for Stimulation of Multiple Formation Intervals.” The '184 patent issued in 2002 and is referred to and incorporated herein by reference in its entirety.
The '184 patent also discloses various techniques for running a bottom hole assembly (“BHA”) into a wellbore, and then creating fluid communication between the wellbore and various zones of interest. In most embodiments, the BHA includes various perforating guns having associated charges. In most embodiments, the BHA is deployed in the wellbore by means of a wireline extending from the surface. The wireline provides electrical signals to the surface for depth control. It also provides electrical signals to the perforating guns for detonation. The electrical signals allow the operator to cause the charges to detonate, at the correct depth or zone, thereby forming perforations.
The BHA also includes a set of mechanically actuated, axial position locking devices, or slips. The slips are actuated through a “continuous J” mechanism by cycling the axial load between compression and tension. In this way, the slips are re-settable.
The BHA further includes an inflatable packer or other sealing mechanism. The packer is actuated by application of a slight compressive load after the slips are set within the casing. Along with the slips, the packer is resettable so that the BHA may be moved to different depths or locations along the wellbore so as to isolate perforations along selected zones of interest.
Each of the various embodiments for a BHA disclosed in the '184 patent includes a means for deploying the assembly into the wellbore, and then translating the assembly up and down the wellbore. Such translation means include a string of coiled tubing, conventional jointed tubing, a wireline, an electric line or a tractor system attached directly to the BHA. In any instance, the purpose of the bottom hole assembly is to allow the operator to perforate the casing along various zones of interest, and then sequentially isolate the respective zones of interest so that fracturing fluid may be injected into the zones of interest in the same trip.
The bottom hole assembly and the formation treating processes disclosed in the '184 patent help to expedite the well completion process. In this respect, the operator is able to selectively set the slips and the packer for perforation and subsequent formation treatment. The operator is able to set the BHA at a first location, fracture or otherwise stimulate a formation, release the BHA, and move it to a new level along the wellbore, all without removing the BHA from the wellbore between stages.
The bottom hole assembly and the formation treating processes disclosed in the '184 patent represent a valuable advance in the art of well completion processes. This process is named “Annular Coiled Tubing FRACturing (ACT-Frac). The ACT-Frac process allows the operator to more effectively stimulate multi-layer hydrocarbon formations at substantially reduced cost compared to previous completion methods.
However, as with previously-known well completion processes, the ACT-Frac process requires the use of expensive surface equipment. Such equipment may include a snubbing unit or a lubricator, which may extend as much as 75 feet above the wellhead. In this respect, the snubbing unit or the lubricator must be of a length greater than the length of the perforating gun assembly (or other tool string) to allow the perforating gun assembly to be safely deployed and removed from the wellbore under pressure. An illustrative lubricator and associated crane arm, wellhead and wellbore are shown in FIG. 1 of co-pending patent application U.S. Patent Publ. No. 2013/0255939. FIG. 1 and the related textual description are incorporated herein by reference.
To avoid the need for a long snubbing unit, it is desirable to fracture the multi-zone formation without the use of a long tool string. Further, it is desirable to complete the well in a multi-zone formation using autonomous perforating guns and ball sealers. Alternatively, it is desirable to perforate a well along multiple zones in a seamless manner using a perforating gun having multiple charges, and using ball sealers.